Re: [PATCH v2 1/7] Adding support to freeze and unfreeze a journal
From: Eric Sandeen
Date: Tue Jan 10 2012 - 15:41:43 EST
On 12/8/11 12:04 PM, Kamal Mostafa wrote:
> From: Surbhi Palande <surbhi.palande@xxxxxxxxxxxxx>
>
> The journal should be frozen when a filesystem freezes. What this means is
> that until the filesystem is thawed again, no new transactions should be
> accepted by the journal. When the filesystem thaws, inturn it should thaw the
> journal and this should allow the journal to resume accepting new
> transactions. While the the filesystem has frozen the journal, the clients of
> the journal on calling jbd2_journal_start() will sleep on a wait queue.
> Thawing the journal will wake up the sleeping clients and journalling can
> progress normally.
>
> An example of the race condition that can happen without this patch is as
> follows:
>
> Say the filesystem is thawed when we begin. Let tx be the time at unit x
>
> P1: Process doing an aio write
> t1) ext4_file_write()
> t2) generic_file_aio_write()
> t3) __generic_file_aio_write()
> // filesystem is not frozen, so we do not block in the next check.
> t4) vfs_check_frozen()
> t5) generic_write_checks()
> ----------------- Prempted------------------
>
> P2: Process that does filesystem freeze
>
> t6) freeze_super()
> t7) sync_filesystem()
> t8) sync_blockdev()
> t9) sb->s_op->freeze_fs() (= ext4_freeze)
> t10) jbd2_journal_lock_updates()
> t11) jbd2_journal_flush()
> // Need to unlock the journal before returning to user space.
> t12) jbd2_journal_unlock_updates()
> // Journal is unlocked and so we can start accepting new transactions now.
>
> // freezing process completes execution. Page cache is now clean and should
> // remain clean till the filesystem is frozen.
> --------------------------------------------
>
> P1: writing process gets the control back
> t13) generic_file_buffered_write()
> t14) generic_perform_write()
> t15) a_ops->write_begin() (= ext4_write_begin)
> t16) ext4_journal_start()
> // New handle is started. We do not block here! Write continues
> // dirtying the page cache while the filesystem is frozen!
Hrm let me think through this a little more; we actually do:
t16) ext4_journal_start()
t17) ext4_journal_start_sb()
t18) handle = ext4_journal_current_handle();
t19) if (!handle) vfs_check_frozen()
t20) ... jbd2_journal_start()
So actually we *do* block new handles, but let *existing* ones
continue (see commits 6b0310fbf087ad6e9e3b8392adca97cd77184084
and be4f27d324e8ddd57cc0d4d604fe85ee0425cba9)
So your assertion that a new handle is started is incorrect
in general, isn't it? So then does the fix seem necessary?
Or, at least, in the fashion below - maybe we need to just make
sure all started handles complete before the unlock_updates?
Or am I missing something...?
-Eric
>
> BugLink: https://bugs.launchpad.net/bugs/897421
> Signed-off-by: Surbhi Palande <surbhi.palande@xxxxxxxxxxxxx>
> Acked-by: Jan Kara <jack@xxxxxxx>
> Reviewed-by: Andreas Dilger <adilger.kernel@xxxxxxxxx>
> Cc: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
> Tested-by: Peter M. Petrakis <peter.petrakis@xxxxxxxxxxxxx>
> Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
> ---
> fs/jbd2/journal.c | 1 +
> fs/jbd2/transaction.c | 42 ++++++++++++++++++++++++++++++++++++++++++
> include/linux/jbd2.h | 7 +++++++
> 3 files changed, 50 insertions(+), 0 deletions(-)
>
> diff --git a/fs/jbd2/journal.c b/fs/jbd2/journal.c
> index 0fa0123..f0170cc 100644
> --- a/fs/jbd2/journal.c
> +++ b/fs/jbd2/journal.c
> @@ -894,6 +894,7 @@ static journal_t * journal_init_common (void)
> init_waitqueue_head(&journal->j_wait_checkpoint);
> init_waitqueue_head(&journal->j_wait_commit);
> init_waitqueue_head(&journal->j_wait_updates);
> + init_waitqueue_head(&journal->j_wait_frozen);
> mutex_init(&journal->j_barrier);
> mutex_init(&journal->j_checkpoint_mutex);
> spin_lock_init(&journal->j_revoke_lock);
> diff --git a/fs/jbd2/transaction.c b/fs/jbd2/transaction.c
> index a0e41a4..340ee35 100644
> --- a/fs/jbd2/transaction.c
> +++ b/fs/jbd2/transaction.c
> @@ -173,6 +173,17 @@ repeat:
> journal->j_barrier_count == 0);
> goto repeat;
> }
> + /* Don't let a new handle start when a journal is frozen.
> + * jbd2_journal_freeze calls jbd2_journal_unlock_updates() only after
> + * the j_flags indicate that the journal is frozen. So if the
> + * j_barrier_count is 0, then check if this was made 0 by the freezing
> + * process
> + */
> + if (journal->j_flags & JBD2_FROZEN) {
> + read_unlock(&journal->j_state_lock);
> + wait_event(journal->j_wait_frozen, (journal->j_flags & JBD2_FROZEN));
> + goto repeat;
> + }
>
> if (!journal->j_running_transaction) {
> read_unlock(&journal->j_state_lock);
> @@ -492,6 +503,37 @@ int jbd2_journal_restart(handle_t *handle, int nblocks)
> }
> EXPORT_SYMBOL(jbd2_journal_restart);
>
> +int jbd2_journal_freeze(journal_t *journal)
> +{
> + int error = 0;
> + /* Now we set up the journal barrier. */
> + jbd2_journal_lock_updates(journal);
> +
> + /*
> + * Don't clear the needs_recovery flag if we failed to flush
> + * the journal.
> + */
> + error = jbd2_journal_flush(journal);
> + if (error >= 0) {
> + write_lock(&journal->j_state_lock);
> + journal->j_flags |= JBD2_FROZEN;
> + write_unlock(&journal->j_state_lock);
> + }
> + jbd2_journal_unlock_updates(journal);
> + return error;
> +}
> +EXPORT_SYMBOL(jbd2_journal_freeze);
> +
> +void jbd2_journal_thaw(journal_t * journal)
> +{
> + write_lock(&journal->j_state_lock);
> + journal->j_flags &= ~JBD2_FROZEN;
> + write_unlock(&journal->j_state_lock);
> + wake_up(&journal->j_wait_frozen);
> +}
> +EXPORT_SYMBOL(jbd2_journal_thaw);
> +
> +
> /**
> * void jbd2_journal_lock_updates () - establish a transaction barrier.
> * @journal: Journal to establish a barrier on.
> diff --git a/include/linux/jbd2.h b/include/linux/jbd2.h
> index 2092ea2..bfa0752 100644
> --- a/include/linux/jbd2.h
> +++ b/include/linux/jbd2.h
> @@ -658,6 +658,7 @@ jbd2_time_diff(unsigned long start, unsigned long end)
> * @j_wait_checkpoint: Wait queue to trigger checkpointing
> * @j_wait_commit: Wait queue to trigger commit
> * @j_wait_updates: Wait queue to wait for updates to complete
> + * @j_wait_frozen: Wait queue to wait for journal to thaw
> * @j_checkpoint_mutex: Mutex for locking against concurrent checkpoints
> * @j_head: Journal head - identifies the first unused block in the journal
> * @j_tail: Journal tail - identifies the oldest still-used block in the
> @@ -775,6 +776,9 @@ struct journal_s
> /* Wait queue to wait for updates to complete */
> wait_queue_head_t j_wait_updates;
>
> + /* Wait queue to wait for journal to thaw*/
> + wait_queue_head_t j_wait_frozen;
> +
> /* Semaphore for locking against concurrent checkpoints */
> struct mutex j_checkpoint_mutex;
>
> @@ -953,6 +957,7 @@ struct journal_s
> #define JBD2_ABORT_ON_SYNCDATA_ERR 0x040 /* Abort the journal on file
> * data write error in ordered
> * mode */
> +#define JBD2_FROZEN 0x080 /* Journal thread frozen along with filesystem */
>
> /*
> * Function declarations for the journaling transaction and buffer
> @@ -1060,6 +1065,8 @@ extern void jbd2_journal_invalidatepage(journal_t *,
> struct page *, unsigned long);
> extern int jbd2_journal_try_to_free_buffers(journal_t *, struct page *, gfp_t);
> extern int jbd2_journal_stop(handle_t *);
> +extern int jbd2_journal_freeze(journal_t *);
> +extern void jbd2_journal_thaw(journal_t *);
> extern int jbd2_journal_flush (journal_t *);
> extern void jbd2_journal_lock_updates (journal_t *);
> extern void jbd2_journal_unlock_updates (journal_t *);
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